1. Field of the Invention
The present invention relates to a color image forming apparatus having a color correcting function and to a method and apparatus for processing a color image.
2. Related Background Art
Various kinds of color image forming apparatuses have been known. Among them, an apparatus using a laser printer in which an image is exposed to a photo-sensitive material by a laser beam and developed to thereby obtain an image is frequently used. Such a laser printer has the advantages that the printing quality is high, the printing speed is high, and the like and, therefore, it is widely used as an output apparatus of, for instance, a color copying machine or the like or as an ordinary printer. In such a laser printer, the modulation for making the light emitting time of the laser correspond to the magnitude of image signal, namely, what is called a pulse width modulation, is executed and an image is formed.
When the pulse width modulation is executed as mentioned above, a plurality of gradation reproducing characteristics as shown by a and b in FIG. 3 can be selected by changing the period of reference pulse. In the reproducing characteristic a shown in the diagram, although the whole image can be sharply reproduced, the high-light portions become dull. In the reproducing characteristic of b, although the gradation of the high-light portions is good, it is difficult to obtain a sharp image as a whole.
In the apparatus for processing a color image by switching the gradation reproducing characteristics as mentioned above, there are the following problems. For instance, when color correcting coefficients are determined on the basis of the reproducing characteristic b as a reference, the high-light portions become dull for the reproducing characteristic a. Therefore, the color reproduction in such portions is not all that could be desired. On the contrary, for example, when color correcting coefficients are decided on the basis of the reproducing characteristic a as a reference, the color reproduction in the high-light portions is deviated.
Such problems are not limited to the apparatus for forming an image by performing pulse width modulation as mentioned above but similarly occur in an apparatus which can change the color image forming conditions when forming a color image or an apparatus in which the gradation reproducing characteristic changes.